1. Field of the Invention
This invention relates to a method for separating .alpha.-nitrocycloalkanone, .alpha.-nitrocycloalkenone or .alpha.-nitrocycloalkadienone from a reaction mixture prepared by the nitrooxidation of a cycloalkene, a cycloalkadiene, or a cycloalkatriene. More particularly, this invention relates to the separation of .alpha.-nitrocycloalkanone (NCA) from a reaction mixture in which a cycloalkene, a cycloalkadiene or a cycloalkatriene is reacted with oxygen and a nitrogen dioxide (hereinafter designated generically as NO.sub.2) by contacting the mixture with a catalyst of a polar compound, and then treating the reaction solution with ammonia.
2. Description of the Prior Art
.alpha.-Nitrocycloalkanone, .alpha.-nitrocycloalkenone and .alpha.-nitrocycloalkadienone (NCA) are raw materials for the preparation of such useful materials as nylon. However, NCA has not heretofore been produced by an economically satisfactory method. Even under optimum process conditions, it has been necessary to use expensive reactants, such as cycloalkanone and acetic anhydride for the production of NCA.
NCA has been prepared by the present inventors by the reaction of cycloalkene, nitrogen dioxide, oxygen and a catalyst of DMSO, DMF, DEF, MPP, DMA, etc. which technique has proven to be substantially simpler and more economical than prior art conventional techniques. At a reaction temperature of -50.degree. C to approximately +20.degree. C, .alpha.-nitrocycloalkylperoxynitrate is produced. The .alpha.-nitrocycloalkylperoxynitrate is denitrated by addition of the reaction mixture to a catalyst containing solution of DMF, whereby NCA is produced.
NCA has also been produced by the one step reaction of admixing nitrogen dioxide and oxygen with a mixture of cycloalkene and a catalyst at -50.degree. C - +50.degree. C whereby an .alpha.-nitrocycloalkylperoxynitrate is formed and denitrated. The reaction mixture contains a solvent, a catalyst, NCA, nitric acid, cycloalkene and nitrated by-products such as 2-nitrocycloalkanol and 2-nitrocycloalkylnitrate and a small amount of other unknown materials and the like.
In the past it has been difficult and troublesome to effectively separate NCA and the catalyst by simple evaporation and distillation techniques in high yield because of the presence of the strong oxidizing agent, i.e. nitric acid, because a complex of nitric acid and the catalyst is formed and because nitric acid and the catalyst are not easily vaporized. In addition, NCA and the catalyst are thermally and chemically unstable. In the past, one method of purification of NCA has been to first extract the by-product nitric acid with water from the reaction mixture so that NCA might easily be separated, and then purify NCA by recrystallization followed by separation. However, recrystallization is not an effective method of purification because the reaction mixture contains other nitrated by-products which have structures similar to NCA. Since the difference in solubility of these by-products in the solvents is small, the recrystallization efficiency is negligible. Moreover, the reaction mixture contains unreacted cycloalkene, while cycloalkane impurities and the like are present in the cycloalkene raw material (for example, 5 - 10% of cyclododecane is present in the cyclododecene raw material). The unreacted cycloalkene and the cycloalkane impurities are relatively soluble in non-polar solvents, while the nitrated by-products are soluble in polar solvents. Thus, in order to remove these two types of impurities, which have different solubilities in different solvents, by recrystallization, it is necessary to use repetative recrystallizations in different solvents to purify the product. However, NCA is relatively soluble in both polar solvents and non-polar solvents, which results in relatively high losses of NCA upon recrystallization. Heretofore, no more than 90% of the NCA has been recovered when NCA is purified by recrystallization. Moreover, the main by-products of the nitrooxidation, 2-nitrocycloalkanol and 2-nitrocycloalkyl nitrate, can be converted to NCA, which makes it highly desirable to recover them in high purity. It is also difficult to recover the by-products by recrystallization. For example, 2-nitrocycloalkanol can be converted to NCA by dehydrogenating the alkanol with an oxidizing agent, while 2-nitrocycloalkylnitrate can be converted to NCA by hydrolyzing the nitrate to yield 2-nitrocycloalkanol and then dehydrogenating the alkanol. Thus, the two by-products as a mixture can be treated to form NCA. However, when the by-products are converted to NCA, NCA should not be present in the reaction mixture because when 2-nitrocycloalkylnitrate is hydrolyzed by mineral acid at high temperatures, NCA undergoes sidereactions.
A need, therefore, exists for a method by which NCA can be obtained in high purity and high yield from reaction solutions. It has been found that since NCA reacts differently with ammonia than other nitrated by-products, NCA can be selectively precipitated as the ammonium salt by treatment with ammonia gas in an organic solvent.